This invention relates generally to data communication and, more particularly, to a method and system for training a modem.
Digital Subscriber Line (DSL) technology allows subscribers to transmit both data and voice signals over the same twisted-pair copper telephone lines. DSL technology utilizes modems at the subscriber""s location and at a remote location, such as a central office of a service provider. These modems facilitate the communication of both voice and data between the subscriber and the central office.
A problem that arises with the use of DSL technology is that the data transmission rates for DSL connections vary depending on a wide variety of factors. For example, the transmission rate in one DSL connection may decrease because of interference caused by another DSL connection, electrical interference caused by lightning, and other variations in the condition of the copper lines. Because of this, the transmission rate achieved over a copper line at one time may not be available over the same copper line at another time. The same subscriber typically cannot receive a consistent transmission rate.
Before the DSL modems at the subscriber""s location and the central office begin transmitting data to one another, DSL chipsets in the modems typically enter a xe2x80x9ctraining mode.xe2x80x9d While in training mode, the chipsets perform handshaking operations and attempt to negotiate rates for data transmissions between the modems. The chipsets typically attempt to negotiate transmission rates for both an upstream direction (away from the subscriber""s location) and a downstream direction (toward the subscriber""s location). The chipsets may attempt to train the modems using certain transmission rates, and if that attempt fails the chipsets try again using different rates.
Conventional DSL chipsets rely heavily on systems using a plurality of tables to train the modems. For example, in DSL modems that use Carrierless Amplitude and Phase (CAP) modulation, the DSL chipsets typically use four-dimensional tables, or tables having four variables, to select transmission rates. The four dimensions in the tables are usually baud rate, signal quality, receiver gain, and desired margin. The baud rate identifies different modulation rates of signals communicated between the modems. The signal quality describes the quality of the signals transmitted between the DSL modems, and lower signal qualities typically represent higher-quality connections. The receiver gain identifies the strength of the signal received by the subscriber""s modem and how much that signal needs to be amplified. The desired margin describes the modems"" ability to correct errors during data transfers before the errors excessively affect the transmission rate. Each entry in these four-dimensional tables also usually includes multiple values. These multiple values identify the upstream and downstream bauds and transmission rates that should be used by the chipsets if the current training attempt fails.
A problem with conventional approaches to training a modem is that these four-dimensional tables take an excessive amount of time to develop. These tables often include a large number of entries, and each entry includes multiple values. Before programming these tables into the memories of the DSL modems, the values are usually determined and then entered into a master table by a programmer. Because there are a large number of values in these tables, determining the values and programming them into the master table may take an excessive amount of time, such as up to several weeks or more.
Another problem with conventional approaches to training a modem is that the tables are also difficult to maintain. For example, in order to upgrade the values in the tables, the updated values are usually determined and then programmed into a master table. It takes time to determine the values and program them into the table. It may also be difficult and time-consuming to locate and replace the old values in the master table.
In addition, the conventional approaches to training a modem usually require a large amount of memory. The values in the four-dimensional tables require storage space. Because of the large number of values in the tables, the tables may require up to 32 kilobytes of memory or more. The DSL modems typically need larger and more expensive memories to store the tables, which also increases the overall size and cost of the modems.
A need has arisen for an improved method and system for training a modem. The present invention provides a method and system for training a modem that addresses shortcomings of prior systems and methods.
In accordance with one embodiment of the present invention, a modem comprises an interface operable to communicate over a subscriber line and a memory operable to store at least one table of acceptable signal qualities. The modem also includes a chipset coupled to the interface. The chipset is operable to train the modem and measure a signal quality after the modem trains. The modem further includes a processor coupled to the chipset and the memory. The processor is operable to select a transmission rate from one of a plurality of bauds, each baud identifying at least one transmission rate. The processor is also operable to communicate the transmission rate to the chipset, where the chipset is operable to attempt to train the modem at the transmission rate. The processor is further operable to receive a measured signal quality from the chipset after the modem trains, and access the table of acceptable signal qualities, the table corresponding to the baud of the transmission rate. In addition, the processor is operable to compare the measured signal quality to at least one acceptable signal quality identified by a provisioned margin in the table to determine if the measured signal quality is acceptable.
In accordance with another embodiment of the present invention, a method for training a modem comprises selecting a transmission rate from one of a plurality of bauds, each baud identifying at least one transmission rate. The method also comprises attempting to train the modem at the transmission rate and measuring the signal quality after the modem trains. The method further comprises accessing a table of acceptable signal qualities, the table corresponding to the baud of the transmission rate. In addition, the method comprises comparing the measured signal quality to at least one acceptable signal quality identified by a provisioned margin in the table to determine if the measured signal quality is acceptable.
Embodiments of the invention provide numerous technical advantages. For example, in one embodiment of the invention, a modem is provided that uses simple tables, such as a plurality of two-dimensional tables, to train the DSL modems. This reduces the number of entries contained in the tables. Also, each entry may contain fewer values, such as only a single value in each entry. This further reduces the overall number of values in the tables. Because there are fewer values in the tables, the values may be determined and programmed into a master table in a shorter amount of time.
Some embodiments of the invention also provide tables that are easier to maintain. Because there are fewer values to update in the tables, determining all of the updated values takes less time. Also, the tables are simpler, so it may be easier to locate and replace the old values in the existing master table.
In addition, some embodiments of the invention require smaller amounts of memory to store the tables. Because the tables have fewer entries and each entry contains fewer values, the tables require less storage space. For example, in one embodiment of the invention, the tables may require approximately 750 bytes of storage space or even less. This allows the DSL modems to use smaller memories to store the tables, which also decreases the overall size and cost of the modems.